From ba773a86f0377abccecd3e398dceb9408bba5a7c Mon Sep 17 00:00:00 2001
From: Jeff Law <jlaw@ventanamicro.com>
Date: Sat, 12 Oct 2024 07:12:53 -0600
Subject: [PATCH] RISC-V] Slightly improve broadcasting small constants into
vectors
I probably spent way more time on this than it's worth...
I was looking at the code we generate for vector SAD and noticed that we were
being a bit silly. Specifically:
li a4,0 # 272 [c=4 l=4] *movsi_internal/1
Followed shortly by:
vmv.s.x v3,a4 # 261 [c=4 l=4] *pred_broadcastrvvm1si/6
And no other uses of a4. We could have used x0 trivially.
First we adjust the expander so that it doesn't force the constant into a
register. In the matching pattern we change the appropriate source constraints
from "r" to "rJ" and the output template is changed to use %z for the operand.
The net is we drop the li completely and emit vmv.s.x,v3,x0.
But wait, there's more. If we're broadcasting a constant in the range
[-16..15] into a vector, we currently load the constant into a register and use
vmv.v.r. We can instead use vmv.v.i, which avoids loading the constant into a
GPR. For that case we again avoid forcing the constant into a register in the
expander and adjust the output template to emit vmv.v.x or vmv.v.i based on
whether or not the appropriate operand is a constant or general purpose
register. So again, we'll drop a load immediate into a scalar for this case.
Whether or not we should use vmv.v.i vs vmv.s.x for loading [-16..15] into the
0th element is probably uarch dependent. The tradeoff is loading the GPR vs
the broadcast in the vector unit. I didn't bother with this case.
Tested in my tester (which tests rv64gcv as a default codegen option). Will
wait for the pre-commit tester to render a verdict.
gcc/
* config/riscv/constraints.md (P): New constraint.
* config/riscv/vector.md (pred_broadcast<mode> expander): Do
not force small integers into GPRs so aggressively.
(pred_broadcast<mode> insn & splitter): Allow splatting small
constants across the vector register directly. Allow splatting
(const_int 0) into element 0 directly.
---
gcc/config/riscv/constraints.md | 5 +++++
gcc/config/riscv/vector.md | 22 ++++++++++++++++------
2 files changed, 21 insertions(+), 6 deletions(-)
diff --git a/gcc/config/riscv/constraints.md b/gcc/config/riscv/constraints.md
index 3ab6d5426223..eb5a0bb75c72 100644
--- a/gcc/config/riscv/constraints.md
+++ b/gcc/config/riscv/constraints.md
@@ -70,6 +70,11 @@
(and (match_code "const_int")
(match_test "ival == 8")))
+(define_constraint "P"
+ "A 5-bit signed immediate for vmv.v.i."
+ (and (match_code "const_int")
+ (match_test "IN_RANGE (ival, -16, 15)")))
+
(define_constraint "K"
"A 5-bit unsigned immediate for CSR access instructions."
(and (match_code "const_int")
diff --git a/gcc/config/riscv/vector.md b/gcc/config/riscv/vector.md
index 92e3061c7f85..a21288f7af2a 100644
--- a/gcc/config/riscv/vector.md
+++ b/gcc/config/riscv/vector.md
@@ -2095,6 +2095,16 @@
emit_move_insn (tmp, gen_int_mode (value, Pmode));
operands[3] = gen_rtx_SIGN_EXTEND (<VEL>mode, tmp);
}
+ /* Never load (const_int 0) into a register, that's silly. */
+ else if (operands[3] == CONST0_RTX (<VEL>mode))
+ ;
+ /* If we're broadcasting [-16..15] across more than just
+ element 0, then we can use vmv.v.i directly, thus avoiding
+ the load of the constant into a GPR. */
+ else if (CONST_INT_P (operands[3])
+ && IN_RANGE (INTVAL (operands[3]), -16, 15)
+ && !satisfies_constraint_Wb1 (operands[1]))
+ ;
else
operands[3] = force_reg (<VEL>mode, operands[3]);
})
@@ -2111,18 +2121,18 @@
(reg:SI VL_REGNUM)
(reg:SI VTYPE_REGNUM)] UNSPEC_VPREDICATE)
(vec_duplicate:V_VLSI
- (match_operand:<VEL> 3 "direct_broadcast_operand" " r, r,Wdm,Wdm,Wdm,Wdm, r, r"))
- (match_operand:V_VLSI 2 "vector_merge_operand" "vu, 0, vu, 0, vu, 0, vu, 0")))]
+ (match_operand:<VEL> 3 "direct_broadcast_operand" "rP,rP,Wdm,Wdm,Wdm,Wdm, rJ, rJ"))
+ (match_operand:V_VLSI 2 "vector_merge_operand" "vu, 0, vu, 0, vu, 0, vu, 0")))]
"TARGET_VECTOR"
"@
- vmv.v.x\t%0,%3
- vmv.v.x\t%0,%3
+ vmv.v.%o3\t%0,%3
+ vmv.v.%o3\t%0,%3
vlse<sew>.v\t%0,%3,zero,%1.t
vlse<sew>.v\t%0,%3,zero,%1.t
vlse<sew>.v\t%0,%3,zero
vlse<sew>.v\t%0,%3,zero
- vmv.s.x\t%0,%3
- vmv.s.x\t%0,%3"
+ vmv.s.x\t%0,%z3
+ vmv.s.x\t%0,%z3"
"(register_operand (operands[3], <VEL>mode)
|| CONST_POLY_INT_P (operands[3]))
&& GET_MODE_BITSIZE (<VEL>mode) > GET_MODE_BITSIZE (Pmode)"
--
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